docs: def derive_fernet_key(input_key):
      """Derive a 32-bit b64-encoded Fernet key from arbitrary input key."""
      hkdf = HKDF(
          algorithm=hashes.SHA256(),
          length=32,
          salt=salt,
          info=info,
          backend=backend,
      )
      return base64.urlsafe_b64encode(hkdf.derive(force_bytes(input_key)))